NO. 414.] NORMAL RESPIRATION. 465 
similar substances no longer usable and presently cast off from 
the body. In plants the elimination of these products is more 
economically accomplished, for they furnish the foundations 
for the re-synthesis of albuminous compounds. These waste 
substances are removed by transforming them synthetically 
into useful compounds. 
The non-nitrogenous substances which become oxidized are 
the fats and oils, the starches and sugars. The oxidation may 
first convert the hydrocarbons into carbohydrates, with the 
liberation of energy and the formation of by-products, carbo- 
hydrates and by-products then becoming still further oxidized 
with the liberation of still more energy. While respiration is 
going on, the other functions also in operation may involve the 
use, with chemical change, of some of each substance produced 
in respiration, and the formation in the cell of other substances 
not the products of respiration at all. It is therefore evident 
that to ascertain the material products of respiration is hardly 
less difficult than to determine the amount of energy liberated. 
Since each process is normal only when accompanied by all 
the processes going on at the same time, it is impossible to 
isolate any physiological process for purposes of study. The 
products of one set of chemical activities in the living body 
may enter wholly or partially, simultaneously or successively, 
into other chemical activities. The end products can be recog- 
nized and measured with comparative ease, but to tell exactly 
where or how they are formed is much more difficult and not 
now entirely possible. 
Water and carbon-dioxide gas are the chief products of the 
Physiological, as also of all other forms of combustion of car- 
bon-containing bodies. They are formed whenever a sufficient 
amount of oxygen is united with the higher carbon compounds. 
In organisms living under such conditions that the air can 
penetrate to all their parts, enough oxygen will always be 
present for such complete decomposition. The oxygen does not 
unite of itself with the combustible compound, for even if active 
(nascent) oxygen is present at all, which seems improbable,’ 
UT daa W. Pflanzen-Physiologie, zte Auflage, Bd. i (1897), p. 554- Pyst 
ants, translated by Ewart, vol. i (1900), pp. 545, 546. 
